We must now return to notice the new development which was taking place while the Christians were erecting their first basilicas in Rome. Intelligent builders in that city were scarce, and architectural styles had become corrupted—a result to which the prevailing practice of destroying ancient monuments and transferring their materials to new buildings for reuse had largely contributed. But, while Rome was languishing, a new era was beginning to dawn for ancient Byzantium, to which Constantine transferred the seat of the empire in the fourth century. Under him the new capital—situated upon the highway of commerce between East and West—grew rapidly in importance. Architecture kept pace with the other developments, but it was carried out under new conditions. Some of the fundamental principles of construction, as well as the art of decoration by mosaics and marble, were adopted from Rome; moreover Constantine, with the view of lowering the importance of the old capital as a rival, carried off from the principal Roman buildings numbers of columns, capitals, and such other architectural ornaments as could be reused in his Byzantine undertakings; but many of his architects, as well as the majority of the artisans he employed, were of Greek descent, hailing from Asia Minor and the East. Byzantium, too, by its trade was brought into direct contact with other nations of the far East, so that there sprang up an Oriental taste for brilliance and rich decoration which at once manifested itself in the architecture.

The divergence from the Roman style is readily observable in the church plan. The simple, rectangular, three-aisled basilica was almost unknown in Byzantium, where its place was taken by a square, vaulted building. In approaching a typical Byzantine church, such as that of Hagia Sophia at Constantinople, or S. Mark's, Venice, the spectator's eye is attracted by the broken sky-line formed by a series of roof-domes, so different from the uninterrupted line of the old basilica roof. The dome, in fact, was the distinguishing feature of Byzantine architecture; and its constant use, for the purpose of roofing over the spaces, had much to do with the radical change of plan from the long rectangle to the square, or Greek-cross form of building.

The Byzantine dome was carried upon four arches enclosing a square, as shown in the diagram, the triangular spaces between the circular dome and the arches being filled in with "pendentives," upon which the dome really rests. It will be seen that each course of masonry forming the pendentives is kept in position by reason of its convexity, so that the dome (shown by the dotted lines) rests securely upon the upper course, at the level of the crown of the arches—i.e. upon the four pendentives.

The most magnificent example of the Byzantine style is the great church at Constantinople, built during the reign of Justinian by Anthemius of Tralles and Isidorus of Miletus, A.D. 532-538, and dedicated to Hagia Sophia, the Holy Wisdom, more commonly, though incorrectly, called the church of S. Sophia. The main building is roofed over by a great central dome, 107 feet in diameter, lighted by a ring of forty small arched windows ranged round the base. The spaces on the east and west are covered by half-domes, which in turn cover semicircular apses. Both the half-domes and the apses are lighted by rings of windows, for upon these roof-openings the whole interior largely depends for light. The weight of the roof is almost entirely carried upon the massive piers which divide the aisles into three bays; so that the whole of the vast nave, measuring more than 200 feet in length and t00 feet in breadth, is unobstructed by columns or piers of any kind. Though differing essentially from the long, wooden-roofed basilicas of Rome, the plan of H. Sophia bears a striking resemblance to that of the basilica of Constantine in the Forum.

The vast unobstructed nave, roofed over with dome upon dome, culminating in the great central vault; the numerous windows, at all heights, vying with the arcades of arches to confuse the eye and thus enlarge the apparent size of the great hall ; the precious marble sheathings of the walls, the rich and delicately carved capitals, and the wonder and wealth of the mosaics, undimmed by the lapse of centuries, with which the vaults are encrusted,-these all combine to make the interior of this vast building one of the most impressive and most harmonious of the triumphs of architecture.

The many influences which were at work on Byzantine architecture resulted in a great variety of plans. At Ravenna, for example, where the art of Rome mingled with that of Byzantium, we have seen that in some of the basilicas—e.g. S. Apollinare in Classe and S. Apollinare Nuovo —the Roman type of building was clothed with details of Oriental character. But other churches differed radically from these. The baptistery of S. John, the surviving portion of a basilica of the fifth century, shows a simple octagonal plan. Octagonal also, but more complicated, is the exquisite church of S. Vitale, where the central dome is carried upon eight piers, between each of which is a semicircular niche or apse; around these is carried an aisle bounded by octagonal walls. The general disposition of the central portion is suggestive of the Pantheon with its eight niches, and is, indeed, almost identical with the temple of Minerva Medica at Rome.

Little attention was paid to the architectural treatment of the exteriors; but the richness of the interiors of the churches of the Byzantine style gives them an interest and a beauty hardly surpassed by buildings of any age. The vaulted system of construction which was adopted produced unbroken expanses of wall and ceiling, which were disturbed very little by projections or moldings—smooth surfaces upon which a decorative effect was gained by means of mosaics. Figure-sculpture and painting had become almost lost arts at this time, and the drawings of the mosaic-workers were rudely simple ; but the materials with which the artists worked their symbolical glass-pictures atoned for much that was lacking in the design, and imparted marvelous beauty and splendor to the simple lines of the architecture. The custom—which originated in Rome—of encrusting the lower walls and the piers with precious marbles, and of enriching the floors with elaborate marble pavements of opus Alexandrinum, contributed to the general effect of splendor and brilliance.

There was much rich carving also of the marble surfaces. The undersides of the arches and the spandrils, or triangular spaces between them, were covered with delicately incised pat-terns; the capitals of the columns were exquisitely carved in crisp low relief, with symbolical emblems, leaf-decoration, etc., and with incised basket-work patterns. Sometimes the volutes and other features of the classical architecture of Rome were suggested, but the general form was similar to the illustration on p. 88.

Above the capital was the impost-block, or dosseret, which we noticed at Ravenna—a very familiar feature in Byzantine work, and probably a reminiscence of the fragmentary entablature of the architecture of the Romans.

Like the Parthenon in the midst of the architecture of Greece, the great church of Hagia Sophia remains unrivalled by any building of its class. Further west, the most beautiful result of the influence of Byzantium is the church of S. Mark at Venice. The original church, which stood where S. Mark's now stands, was destroyed by fire. In 977 the new building was begun, and was probably carried out mainly by builders from Byzantium, for, with the exception of minor de-tails of later date, it is purely Byzantine in character. Those who have not visited Venice will be familiar, from photographs and drawings, with the form of S. Mark's richly encrusted front, a facade worthy of the picture which Ruskin draws in his " Stones of Venice " :—" a multitude of pillars and white domes, clustered into a long low pyramid of colored light ; a treasure-heap, it seems, partly of gold, and partly of opal and mother-of-pearl, hollowed beneath into five great vaulted porches, ceiled with fair mosaic, and beset with sculpture of alabaster, clear as amber and delicate as ivory. And round the walls of the porches there are set pillars of variegated stones,

jasper and porphyry, and deep-green serpentine spotted with flakes of snow, and marbles, that half refuse and half yield to the sunshine, Cleopatra-like, ' their bluest veins to kiss,'—the shadow, as it steals back from them, revealing line after line of azure undulation, as a receding tide leaves the waved sand; their capitals rich with interwoven tracery, rooted knots of herbage, and drifting leaves of acanthus and vine, and mystical signs, all beginning and ending in the Cross; and above them, in the broad archivolts, a continuous chain of language and of life—angels, and the signs of heaven, and the labors of men, each in its appointed season upon the earth; and above these, another range of glittering pinnacles, mixed with white arches edged with scarlet flowers,—a confusion of delight, amidst which the breasts of the Greek horses are seen blazing in their breadth of golden strength."

The Byzantine style has had little influence upon the architecture of Western Europe. In Greece and Russia it became, and has continued to be, the recognized style for buildings of the Greek Church, though it has naturally received many modifications. When Constantinople fell into the hands of the Turks (1453), the old architecture was revived, and was applied to the building of mosques, so that it was destined to exert considerable influence upon the building forms of the Moslems.